1. Field of the Invention
The present invention relates to an anode, a method of manufacturing the anode, a battery including the anode, and a method of manufacturing the battery.
2. Description of the Related Art
In recent years, portable electronic devices such as camera-integrated VTRs (videotape recorders), digital steel cameras, cellular phones, personal digital assistances or laptop computers are widely used, and size and weight reduction in the portable electronic devices and an increase in longevity of the portable electronic devices have been strongly demanded. Accordingly, as power sources for the portable electronic devices, the development of batteries, specifically lightweight secondary batteries capable of obtaining a high energy density have been promoted.
Among them, a secondary battery (a so-called lithium-ion secondary battery) using insertion and extraction of lithium for charge-discharge reaction holds great promise, because the secondary battery is capable of obtaining a larger energy density, compared to a lead-acid battery or a nickel-cadmium battery. In the lithium-ion secondary battery, a carbon material is widely used as an anode active material.
Recently, with enhancement of performance in portable electronic devices, a further improvement in capacity is desired, so it is considered to use tin, silicon or the like as an anode active material instead of a carbon material (for example, refer to U.S. Pat. No. 4,950,566). It is because the theoretical capacities of tin (994 mAh/g) and silicon (4199 mAh/g) are much larger than the theoretical capacity of graphite (372 mAh/g), so a remarkable improvement in battery capacity is expected.
However, in a lithium-ion secondary battery with a higher capacity, an anode active material into which lithium is inserted during charge has high activity, so there is an issue that an electrolytic solution is easily decomposed, and lithium is inactivated. Therefore, when charge and discharge are repeated, charge-discharge efficiency declines, thereby it is difficult to obtain sufficient cycle characteristics.
Therefore, to improve battery characteristics such as cycle characteristics, an electrolytic solution including an alkali metal salt of hexafluoroantimonic acid, an alkali metal salt of hexafluorotantalic acid, a fluorocomplex salt such as lithium hexafluoroniobate, lithium hexafluorovanadate, lithium tetrafluoroferrate, lithium pentafluorotitanate or lithium pentafluorozirconate, or a halogeno complex salt such as alkali metal periodic acid salt is used (for example, refer to Japanese Unexamined Patent Application Publication Nos. S58-204478, S63-310568, H03-152879, H06-290808, 2002-047255 and 2003-142154).
Moreover, to improve cycle characteristics, it is considered to form an inert coating on a surface of an anode active material through the use of a lithium compound such as lithium fluoride or lithium carbonate, or an oxide such as silicon oxide or aluminum oxide (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2004-327211, H07-302617, H10-255800, H11-135153, 2005-026230, 2005-142156, 2005-166469 and 2006-185728).